First layer on a foundation

[asnoop20]

Hi all,

I've just filled a 10M trench foundation, which has taken 2 days by mixer. I set pegs but there does seem to be a little variance here and there between them.

1. I read that I should start to build the corners first, but what happens if somewhere in the middle there's a bit of a hump and the brick/block won't fit (i.e no mortar)? Or can/should the first layer be something like 20mm mortar height? Also being 10M that's prone to string sag?

2. when there's no exit point for water drainage is it enough to use gaps in the mortar then protect with dpm and gravel to prevent soil build up?

3. I'll be using concrete blocks which won't be visible on my side (the retained soil), so is it worth DPM'ing the rear or should I just let it get wet?

Cheers, Andrew

 

[Deleted member 323070]

Hi all,

I've just filled a 10M trench foundation, which has taken 2 days by mixer. I set pegs but there does seem to be a little variance here and there between them.

Set the corner blocks first, dry, perhaps on 10mm thick pieces of timber or whatever is to hand.
I use a string line level over that kind of distance.
Then, if neccessary, I'll set one or two midway blocks, dry again, to get precise levels with long levels or pices of timber.

1. I read that I should start to build the corners first, but what happens if somewhere in the middle there's a bit of a hump and the brick/block won't fit (i.e no mortar)? Or can/should the first layer be something like 20mm mortar height? Also being 10M that's prone to string sag?

By setting out the corner blocks, and perhaps the midpoint blocks, you get a good idea of the levels all along the 10m.
When setting subsequent layers in the middle, use a "tingle", a small piece of card or paper, folded to enclose the string line, and set, again dry ,at the midpoints. Hold it in place by placing it on the top of a block, held in place by a weight on top. It supports the stringline along the whole length.
If necessary cut a slice of the block to allow a thickness of mortar.

2. when there's no exit point for water drainage is it enough to use gaps in the mortar then protect with dpm and gravel to prevent soil build up?

Use short pieces of plastic pipe.

3. I'll be using concrete blocks which won't be visible on my side (the retained soil), so is it worth DPM'ing the rear or should I just let it get wet?

It could look unattractive with the damp. It could affect the colour, and it could allow moss to grow.
The DPM should allow the escape of any water build-up. So you might not exclude entirely the damp wall, but it will reduce it.

 

[^woody^]

How are you leveling the two ends? You'll need a datum at mid point too, so use the same method

Yes you'll need to hold the line up to prevent it sagging - known as a tingle. I use £10 notes.

Get over any humps and dips in the concrete with cut blocks, blocks flat, or blend some bricks in, or bigger joints. Do this on the first course where it's not seen, the second course should be normal and level.

You don't have DPM or DPC in retaining or garden walls.

 

[asnoop20]

I'm still not sure how string resolves the levels all over the 10m, I'd have to set the level by say a block and a brick and then drag a block all the way along making sure it doesn't come too close to the string (where there'd be little mortar)? Or alternatively use a significant amount of mortar on the first layer in hope that it's enough to get over variance.

Or cut blocks as you say woody, but I don't want to be cutting a huge amount. I think I'll set the line with a block & 10mm timer and then drag a block and see that it doesn't come close to the line first.

Not sure how the level is set, I have a laser level but can't see how it's useful over this distance. For the foundation I set the string then manually held a long spirit level. Frankly it's a bit **** and I'll find out whether it's held up tomorrow when I can step on the foundation.

 

[Deleted member 323070]

Not sure how the level is set, I have a laser level but can't see how it's useful over this distance. For the foundation I set the string then manually held a long spirit level. Frankly it's a bit **** and I'll find out whether it's held up tomorrow when I can step on the foundation.

Use a string line level.

 

[noseall]

Yes you'll need to hold the line up to prevent it sagging - known as a tingle. I use £10 notes.

Or pull the line that taut you get a G flat or an E minor if you deadly accuracy.

 

[asnoop20]

String line level, genius

 

[^woody^]

Or pull the line that taut you get a G flat or an E minor if you deadly accuracy.

I like the sound of that

 

[noseall]

Use a string line level.
View attachment 381466

Not a great deal of use on a long line, or even if there is a modest breeze.
We pour the footings conc' then laser datum pegs on the corners, set at existing DPC height.

 

[noseall]

Hi all,

I've just filled a 10M trench foundation, which has taken 2 days by mixer. I set pegs but there does seem to be a little variance here and there between them.

1. I read that I should start to build the corners first, but what happens if somewhere in the middle there's a bit of a hump and the brick/block won't fit (i.e no mortar)? Or can/should the first layer be something like 20mm mortar height? Also being 10M that's prone to string sag?

2. when there's no exit point for water drainage is it enough to use gaps in the mortar then protect with dpm and gravel to prevent soil build up?

3. I'll be using concrete blocks which won't be visible on my side (the retained soil), so is it worth DPM'ing the rear or should I just let it get wet?

Cheers, Andrew

Always cut the masonry to suit.

We'd rather muck up than cut down.

 

[asnoop20]

How about I set the laser and use a ruler at one end then rotate the laser to point to the opposing end of the foundation, wait until dusk/dark (as it's too far in daylight), and ruler that end. I'll then know which end is higher?

I can probably just walk the foundation as well during the evening dragging the ruler along to see what variance there is. Highest point wins.

 

[noseall]

How about I set the laser and use a ruler at one end then rotate the laser to point to the opposing end of the foundation, wait until dusk/dark (as it's too far in daylight), and ruler that end. I'll then know which end is higher?

I can probably just walk the foundation as well during the evening dragging the ruler along to see what variance there is. Highest point wins.

If you have a bulge in the middle, then muck up on the corners to compensate, if your coursing up allows you to do so.

 

[asnoop20]

So the damage is in.

Left corner to right corner

0, -45mm, -80mm, -60mm, -50mm, -70mm, -50mm, -40mm, -40mm

So I've over-cooked it on the left hand side then there was a dip and then it's reasonably flat.

I'm thinking then I lay the right hand side as normal, then cut blocks to make the left hand side match, string across, then everything else gets additional mortar. Sound like a plan? It means that -80mm is going to have 50mm of mortar (I'll make it strong though).

 

[Old Salt]

Are those measurements correct?. 80 centimetres is a little more than slight variance its over 3 inches!

No doubt the experts will have a solution.

 

[asnoop20]

Yeah, but that's the worst point at the left corner, an outlier. The rest has variance of 40mm.